A: There are a number of reasons this may be happening, and future updates to the CDCEther driver may solve some of them. A user on the Linux-USB-user mailing list noticed that on at least one occasion data sent to the modem from upstream by the cable provider has triggered it. Also, the modem itself is very sensitive to power interruptions and can lose the connection if this occurs. The fix is to run ifdown ethX, where ethX is the ethernet interface (eth0, eth1 etc.) to clear out any remaining settings that are hung, then remove the module with rmmod CDCEther, reinsert the CDCEther module and then ifup ethX . A reboot may be necessary if this doesn't fix the problem. If none of these work you probably have a real service interruption.

A: No. The multicast message is pertaining to Multicast support in the kernel, which is optional and not necessary for the proper functioning of this modem. The message about 'Ethernet Information' is a design bug in the modem and can be ignored. As for the 'Imperfect filtering support,' to quote Brad Hards:

"
This is a bit difficult to explain - I assume that you know what multicasting is - when you join a multicast group, this can be handled by the networking device so that other multicast traffic doesn't cause interrupts. That is called 'perfect filtering.' However sometimes the number of multicast addresses exceeds the number of filters that you have. This leads to 'imperfect filtering,' which can cut down the number of interrupts, but you still need to do some work in the networking stack. Then you get to the typical cable modem implementation,
and there is not filtering at all. Every multicast packet goes to the host to be filtered. This doesn't normally matter though, because the cable modem is a point to point link.
"

Q: I'm still having problems, or I'm unusually curious...is there a way that I can get more information about what the device is up to?

A: Yes, there is. The manufacturer hard-wired an http server for status and configuration purposes into the modem itself. It seems to have been designed for troubleshooting by cable tecnician staff, but you can access the 4100 and 4200 models by directing your web browser to http://192.168.100.1. You will need to kill your firewall if you have one running prior to doing this. You can see statistics, logs and some other miscellaneous info at this address.